make sure your script has been reviewed / approved
ensure everyone know what their role is (director, cinematographer, sound, etc.)
ensure that the cinematographer has signed out the equipment and tested ALL of the camera equipment and charged the batteries. If it will be a long shoot, see Mr. Kingsland and get an extra camera battery if one is available.
ensure the sound person has tested ALL of the sound equipment, has spare batteries, and can set everything up without assistance
ensure your actors have copies of the scripts and have reviewed / practiced their lines
check your location and see if there are any changes needed (i.e. backlighting, etc.)
Get lights, window coverings, light difusers, greenscreen, etc. if needed.
plan your FOOD. Be healthy. 'Gone with the Wind' was not made on pizza and coke.
finally, get yourself into the right frame of mind. If someone is constantly fooling around, they need to be removed from your group.